The present invention relates in general to improvements in a vitrified grinding wheel including an annular core body which is constituted principally by a synthetic resin, and an abrasive layer which has a vitrified abrasive structure and which is disposed radially outwardly of the annular core body.
There is known a vitrified grinding wheel having in its center a mounting hole which is to be fitted onto a wheel spindle of a grinding machine, and including an annular core body and an abrasive layer which is disposed radially outwardly of the annular core body and which includes a multiplicity of abrasive grains held together by a vitrified bond. Commonly, the annular core body is provided by a carbon steel, an aluminum alloy or a synthetic resin, while the abrasive layer is provided by standard abrasive grains (such as silicon carbide grains and alumina grains) or super abrasive grains (such as diamond grains and CBN grains). As an example of such a vitrified grinding wheel, there is a segment-chip-type grinding wheel in which the abrasive layer is provided by a plurality of abrasive segment chips bonded to an outer circumferential surface of the annular core body.
When a cylindrical workpiece requires to be finished at its surface by the grinding wheel such that the surface of the hard coating is smoothed to have a roughness Ra (arithmetical mean deviation of profile), for example, of 0.2–0.5 μm, at least one of the cylindrical workpiece and the grinding wheel is axially moved relative to the other, with the workpiece and the grinding wheel being both rotated about their respective axes which are held in parallel with each other. In this instance, there is a case where the grinding wheel suffers from its self-oscillation induced, for example, by run out (eccentricity), out of balance, or out of round of the grinding wheel itself, so that chatter marks such as wavy line and spirals are undesirably formed on the surface. Such undesirable marks are not notable in a measurement for the surface roughness, namely, do not affect the measured value of the surface roughness. However, where the ground workpiece is a roll used for a rolling mill, such marks are inconvenient, because the marks on the surface of the roll are likely to be transferred onto a surface of a plate or sheet rolled by the roll, or are likely to cause an oscillation of the roll, resulting in a reduced yield rate in the operation of the rolling mill. The tendency of the self-oscillation of the grinding wheel is increased, particularly, where the roll is formed of a hard material such as a ceramic material and a high-speed tool steel, and is ground by the abrasive layer provided by the super abrasive grains such as diamond grains and CBN grains.
In view of the above-described problem, there is proposed a vitrified grinding wheel capable of preventing the formation of the chatter marks in a grinding operation, as disclosed in JP-A-H05-285848 (publication of unexamined Japanese Patent Application laid open in 1993). The annular core body of the disclosed grinding wheel has an elastic modulus of 1500–5000 kgf/mm2 as measured in a radial direction of the annular core body, so that the core body having the elasticity serves to absorb oscillation of the grinding surface of the grinding wheel which could be induced by run out (eccentricity), out of balance, or out of round of the grinding wheel. Therefore, the formation of the chatter marks on the ground surface in the grinding operation is advantageously prevented.
However, where the annular core body of the vitrified grinding wheel is provided by a resinoid structure in which aggregate particles such as silicon carbide and alumina are held together by a resin bond, there is a problem that abrasive segment chips 50 (cooperating with each other to constitute an abrasive layer) have cracks 52, as shown in FIG. 5, after its long-time use or storage. An extensive study by the present inventor revealed that the problematic cracks are caused by change in the volume of the annular core body which is subjected to a cutting fluid used in a grinding operation or moisture contained in the atmosphere.